In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Current laser-based lithographic systems frequently rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer in order to facilitate its removal. Because ablation produces airborne debris, ablation-type plates must be designed with imaging byproducts in mind; for example, the plate may be designed so as to trap ablation debris between layers, at least one of which is not removed until after imaging is complete.
Dry plates, which utilize an oleophobic topmost layer of fluoropolymer or, more commonly, silicone (polydiorganosiloxane), exhibit excellent debris-trapping properties because the topmost layer is tough and rubbery; ablation debris generated thereunder remains confined as the silicone or fluoropolymer does not itself ablate. Where imaged, the underlying layer is destroyed or de-anchored from the topmost layer. A common three-layer plate, for example, is made ready for press use by image-wise exposure to imaging (e.g., infrared or “IR”) radiation that causes ablation of all or part of the central layer, leaving the topmost layer de-anchored in the exposed areas. Subsequently, the de-anchored overlying layer and the central layer are removed (at least partially) by a post-imaging cleaning process—e.g., rubbing of the plate with or without a cleaning liquid—to reveal the third layer (typically an oleophilic polymer, such as polyester).
Ablation debris remaining in the newly formed gap between layers includes pyrolytic remnants of the imaging layer, as well as fragments of the overlying and underlying layers. Although these layers are not ablated in the bulk sense—i.e., ablation is limited to the interfacial areas near the imaging layer—they do shed material in response to the heat generated by pyrolysis of the imaging layer. Silicone debris can be particularly resistant to removal. In many printing systems, application of the ink itself (during the print “make-ready” process, which involves several preliminary cycles of inking and printing) removes any debris that persisted through the cleaning step.
This is possible, however, only if the ink contains a solvent capable of entraining or at least partially dissolving the debris. Many inks in current use are solventless inks, such as those curable by ultraviolet (“UV”) radiation. UV-curable inks are considered “100% solid systems” in that they contain only pigment and acrylic monomers; although they are not dry (having, instead, a paste-like viscosity), they do not contain solvents. A typical UV-curable waterless ink may consist of blue pigment 16%, carbon black 4%, epoxy acrylate resin 30%, fatty acid modified epoxy acrylate 25%, monomer viscosity modifier 8%, benzophenone initiator 8%, co-initiator 3%, photosynergist 4% and wax 2%.
Such inks, unfortunately, do not contribute to plate cleaning. A press operator will complain of “poor ink-up”: the exposed regions of the plate are slow to accept ink due to remnant silicone. Thus, there is a need for a cleaning regimen that accommodates solventless inks, but which does not impose undesirable environmental consequences, inconvenience or expense.